Nucleophilic Nucleophilic Aromatic Substitution Aromatic Substitution Aryl Halides & Aryl Halides & Benzyne Benzyne Cl Cl OH 1. 1. NaOH NaOH, H , H 2 O 370°C 370°C 2. H 2. H + Not practical. Not practical. Not sufficiently reactive. Not sufficiently reactive. Chlorobenzene is very unreactive with nucleophiles Cl C + Cl S N 1 empty sp 2 orbital Aryl Cation SN1 not reasonable because: 1) C—Cl bond is strong; therefore, ionization to a carbocation is a high-energy process 2) aryl cations are highly unstable Reasons for Low Reactivity Reasons for Low Reactivity Reasons for Low Reactivity Reasons for Low Reactivity S N 2 not reasonable because ring blocks attack 2 not reasonable because ring blocks attack of of nucleophile nucleophile from side opposite bond to from side opposite bond to leaving group leaving group nitro-substituted aryl halides nitro-substituted aryl halides do do undergo undergo nucleophilic aromatic substitution readily nucleophilic aromatic substitution readily But... But... Cl Cl NO NO 2 + Na NaOCH CH 3 CH CH 3 OH OH 85°C 85°C OCH CH 3 NO NO 2 + Na NaCl Cl (92%) (92%) especially when nitro group is ortho and/or especially when nitro group is ortho and/or para to leaving group para to leaving group Effect of nitro group is cumulative Effect of nitro group is cumulative Cl Cl 1.0 1.0 Cl Cl NO NO 2 7 x 10 7 x 10 10 10 Cl Cl NO NO 2 NO NO 2 O 2 N 2.4 x 10 2.4 x 10 15 15 Cl Cl NO NO 2 NO NO 2 too fast to measure too fast to measure
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Not practical.Not practical.Not sufficiently reactive.Not sufficiently reactive.
Chlorobenzene is very unreactive withnucleophiles
Cl
C
+ Cl
SN1
empty
sp2 orbital
Aryl Cation
SN1 not reasonable because:
1) C—Cl bond is strong; therefore, ionizationto a carbocation is a high-energy process
2) aryl cations are highly unstable
Reasons for Low ReactivityReasons for Low Reactivity Reasons for Low ReactivityReasons for Low Reactivity
SSNN2 not reasonable because ring blocks attack2 not reasonable because ring blocks attackof of nucleophile nucleophile from side opposite bond tofrom side opposite bond toleaving groupleaving group
especially when nitro group is ortho and/orespecially when nitro group is ortho and/orpara to leaving grouppara to leaving group
Effect of nitro group is cumulative Effect of nitro group is cumulative
ClCl
1.01.0
ClCl
NONO22
7 x 107 x 101010
ClCl
NONO22
NONO22OO22NN
2.4 x 102.4 x 101515
ClCl
NONO22
NONO22
too fast to measuretoo fast to measure
QuestionQuestionWhich compound will react faster with NaOCHWhich compound will react faster with NaOCH33in methanol (50in methanol (50ooC)?C)?
A)A) B)B)
C)C) D)D)
follows second-order rate law:follows second-order rate law:rate = rate = k k [aryl halide][[aryl halide][nucleophilenucleophile]]
inference:inference:both the aryl halide and the both the aryl halide and the nucleophile nucleophile are are involved in rate-determining stepinvolved in rate-determining step
KineticsKinetics
Effect of leaving groupEffect of leaving group
unusual order: F > Cl > Br > Iunusual order: F > Cl > Br > I
XX
NONO22
XX Relative Rate*Relative Rate*
FF
ClCl
BrBr
II
312312
1.01.0
0.80.8
0.40.4
*NaOCH*NaOCH33, CH, CH33OH, 50°COH, 50°C
••bimolecular rate-determining step in whichbimolecular rate-determining step in whichnucleophile nucleophile attacks aryl halideattacks aryl halide••rate-determining step precedes carbon-halogenrate-determining step precedes carbon-halogenbond cleavagebond cleavage••rate-determining transition state is stabilized byrate-determining transition state is stabilized byelectron-withdrawing groups (such as NOelectron-withdrawing groups (such as NO22))
General Conclusions About MechanismGeneral Conclusions About Mechanism
The Addition-Elimination MechanismThe Addition-Elimination Mechanismof of
Two step mechanism:Two step mechanism:Step 1) Step 1) nucleophile nucleophile attacks aryl halide andattacks aryl halide and
bonds to the carbon that bears the halogenbonds to the carbon that bears the halogen(slow: (slow: aromaticity aromaticity of ring lost in this step)of ring lost in this step)
Step 2) intermediate formed in first step losesStep 2) intermediate formed in first step loseshalidehalide
(fast: (fast: aromaticity aromaticity of ring restored in this step)of ring restored in this step)
How many signals would be observed in the How many signals would be observed in the 11H-H-NMR of the product isolated from the reaction ofNMR of the product isolated from the reaction ofpp-fluoronitrobenzene -fluoronitrobenzene with potassium with potassium methoxidemethoxidein methanol?in methanol?
A)A) 22B)B) 33C)C) 44D)D) 55
QuestionQuestion
Identify the rate law for the addition-eliminationIdentify the rate law for the addition-eliminationmechanism of mechanism of nucleophilic nucleophilic aromaticaromatic
bimolecularbimolecularconsistent with second-consistent with second-order kinetics; first orderorder kinetics; first orderin aryl halide, first orderin aryl halide, first orderin in nucleophilenucleophile
Step 1Step 1
MechanismMechanism
slowslow
•••• OOCHCH33••••
••••
––
NONO22
FF••••
•••• ••••
HH
HH
HH
HH
NONO22
FF••••
•••• ••••HH
HH
HH
HH
•••• ––
•••• OOCHCH33••••
Step 1Step 1
MechanismMechanism
NONO22
FF••••
•••• ••••HH
HH
HH
HH
••••––
•••• OOCHCH33••••intermediate isintermediate is
negatively chargednegatively chargedformed faster whenformed faster whenring bears electron-ring bears electron-withdrawing groupswithdrawing groupssuch as NOsuch as NO22
Stabilization of Rate-Determining IntermediateStabilization of Rate-Determining Intermediateby Nitro Groupby Nitro Group
NN
FF••••
•••• ••••HH
HH
HH
HH
••••
•••• OOCHCH33••••
OO OO••••••••
•••••••• ••••++
––
––
Stabilization of Rate-Determining IntermediateStabilization of Rate-Determining Intermediateby Nitro Groupby Nitro Group
NN
FF••••
•••• ••••HH
HH
HH
HH
••••
•••• OOCHCH33••••
OO OO••••••••
•••••••• ••••++
––
––
NN
FF••••
•••• ••••HH
HH
HH
HH
••••
•••• OOCHCH33••••
OO OO••••••••
•••••••• ••••++
––
––
MechanismMechanism
NONO22
FF••••
•••• ••••HH
HH
HH
HH
••••––
•••• OOCHCH33••••
Step 2Step 2
MechanismMechanism
fastfast
•••• OOCHCH33••••
NONO22
HH
HH
HH
HH
NONO22
FF••••
•••• ••••HH
HH
HH
HH
••••––
•••• OOCHCH33••••
FF••••
•••• ••••••••
––
Step 2Step 2
QuestionQuestion
Which of the structures below is the most stableWhich of the structures below is the most stableresonance structure for the reaction of resonance structure for the reaction of pp--fluoronitrobenzene fluoronitrobenzene with sodium with sodium methoxidemethoxide??
A)A) 1 only1 only B) 2 onlyB) 2 onlyC)C) 3 only3 only D) 1 and 2D) 1 and 2
carbon-halogen bond breaking does not occurcarbon-halogen bond breaking does not occuruntil after the rate-determining stepuntil after the rate-determining step
electronegative F stabilizes negatively electronegative F stabilizes negatively charged intermediatecharged intermediate
Leaving Group EffectsLeaving Group Effects
F > Cl > Br > I is unusual, but consistentF > Cl > Br > I is unusual, but consistentwith mechanismwith mechanism
The Role of Leaving Groups
FNuc
H
N
O O
ClNuc
H
N
O O
BrNuc
H
N
O O
INuc
H
N
O O
Most Stabilized Least Stabilized
QuestionQuestion
Which of the following compounds is the leastWhich of the following compounds is the leastreactive toward reactive toward nucleophilic nucleophilic aromaticaromatic
Six fluorine Six fluorine substituents substituents stabilize negativelystabilize negativelycharged intermediate formed in rate-determiningcharged intermediate formed in rate-determiningstep and increase rate of nucleophilic aromaticstep and increase rate of nucleophilic aromaticsubstitution.substitution.
2-Chloropyridine reacts 230,000,000 times2-Chloropyridine reacts 230,000,000 timesfaster than faster than chlorobenzene chlorobenzene under theseunder theseconditions.conditions.
Nitrogen is more electronegative than carbon,Nitrogen is more electronegative than carbon,stabilizes the anionic intermediate, andstabilizes the anionic intermediate, andincreases the rate at which it is formed.increases the rate at which it is formed.
Nitrogen is more electronegative than carbon,Nitrogen is more electronegative than carbon,stabilizes the anionic intermediate, andstabilizes the anionic intermediate, andincreases the rate at which it is formed.increases the rate at which it is formed.
Aryl Halides Undergo Substitution WhenAryl Halides Undergo Substitution WhenTreated With Very Strong BasesTreated With Very Strong Bases
ClCl
NHNH22KNHKNH22, NH, NH33
––33°C33°C
(52%)(52%)
CHCH33
NHNH22
new new substituent substituent becomes attached to eitherbecomes attached to eitherthe carbon that bore the leaving group orthe carbon that bore the leaving group orthe carbon adjacent to itthe carbon adjacent to it
RegiochemistryRegiochemistry
++
NaNHNaNH22,,NHNH33
––33°C33°C
CHCH33
BrBr
CHCH33
NHNH22
new new substituent substituent becomes attached to eitherbecomes attached to eitherthe carbon that bore the leaving group orthe carbon that bore the leaving group orthe carbon adjacent to itthe carbon adjacent to it
RegiochemistryRegiochemistry
CHCH33
BrBr
++NaNHNaNH22, NH, NH33
––33°C33°C
CHCH33
NHNH22
CHCH33
NHNH22
RegiochemistryRegiochemistry
++
NaNHNaNH22, NH, NH33 ––33°C33°C
CHCH33
NHNH22
CHCH33
NHNH22
CHCH33
ClCl
++
CHCH33
NHNH22
QuestionQuestion
What product will be isolated when What product will be isolated when oo--bromotoluene bromotoluene is treated with NaNHis treated with NaNH22 in NH in NH33??
A)A) B)B)
C)C) D) Both A and BD) Both A and B
Same result using Same result using 1414C labelC label
ClCl**
KNHKNH22, NH, NH33 ––33°C33°C
NHNH22**++
NHNH22
**
(48%)(48%)(52%)(52%)
MechanismMechanism
•••• NNHH22••••
––
Step 1Step 1
HH
HH•••• ••••
HH
HH
ClCl
HH
••••
MechanismMechanism
•••• NNHH22••••
––
Step 1Step 1
HH
HH•••• ••••
HH
HH
ClCl
HH
••••
HH
HH
HH
HHNNHH22••••
HH
•••••••• ClCl••••
••••––
compound formed in this step is called compound formed in this step is called benzynebenzyne
BenzyneBenzyne
HH
HH
HH
HH
Benzyne Benzyne has a strained triple bond.has a strained triple bond.It cannot be isolated in this reaction, but isIt cannot be isolated in this reaction, but isformed as a reactive intermediate.formed as a reactive intermediate.
Benzyne - A Reactive MoleculeWith an Abnormal π-Bond
Benzyne has a reactive triple bond.It cannot be isolated in this reaction, but isformed as a reactive intermediate.
H
H
H
H
Benzyne
H
H
H
H
2pZ-2pZ
! Bond
H
H
H
H
sp2-sp2
! Bond
Benzyne - A Reactive Aromatic MoleculeWith An Abnormal, In-Plane π-Bond
C
C
R
R
'Normal' C-C Triple Bond
C
CC
CCCH
H H
H
Benzyne C-C Triple Bond
overlapping sp2 orbitals
poor overap results
in a weak, reactive bond
MechanismMechanism
•••• NNHH22••••
––
Step 2Step 2
HH
HH
HH
HH
MechanismMechanism
•••• NNHH22••••
––
Step 2Step 2
HH
HH
HH
HH
HH
HH
HH
HH
NNHH22
––••••
••••
Angle strain is relieved. The two Angle strain is relieved. The two spsp-hybridized-hybridizedring carbons in ring carbons in benzyne benzyne become become spsp22 hybridized hybridizedin the resulting anion.in the resulting anion.
MechanismMechanism
Step 3Step 3
HH
HH
HH
HH
NNHH22
––••••
••••
NNHH22••••HH
MechanismMechanism
•••• NNHH22••••
––Step 3Step 3
HH
HH
HH
HH
NNHH22
––••••
••••
NNHH22••••HH
HH
HH
HH
HH
HH NNHH22••••
QuestionQuestion
Which of the following compounds give a singleWhich of the following compounds give a singlebenzyne benzyne intermediate on reaction withintermediate on reaction withsodium amide?sodium amide?
A)A) 1 only1 only B) 1 and 3B) 1 and 3C)C) 3 only3 only D) 1 and 2D) 1 and 2
Diels-Alder Diels-Alder Reactions of Reactions of BenzyneBenzyne
Other Routes to Other Routes to BenzyneBenzyne
Benzyne Benzyne can be prepared as a reactivecan be prepared as a reactiveintermediate by methods other than treatment ofintermediate by methods other than treatment ofchlorobenzene chlorobenzene with strong bases.with strong bases.Another method involves loss of fluoride ionAnother method involves loss of fluoride ionfrom the from the Grignard Grignard reagent of 1-bromo-2-reagent of 1-bromo-2-fluorobenzene.fluorobenzene.
Other Routes to Other Routes to BenzyneBenzyne
••••
BrBr
FF••••
••••
Mg, THFMg, THF
heatheat ••••
MgMgBrBr
FF••••
••••
FFMgMgBrBr++
Benzyne Benzyne as a as a DienophileDienophile
Benzyne Benzyne is a fairly reactive is a fairly reactive dienophiledienophile, and, andgives gives DielsDiels-Alder adducts when generated in the-Alder adducts when generated in thepresence of conjugated presence of conjugated dienesdienes..
The Diels-Alder Reaction Revisited
BA
X Y
A
X
B
Y
cycloaddition
diene dienophile cycloadduct
H
H
H
H
isoprene maleic anhydride cycloadduct
O
O
O
H3C H3C
O
O
O
100°C
Electron-Deficient AlkynesBehave as Dienophiles
butadiene but-3-yn-2-one cycloadduct
120°C
CH3O
HH
CH3
O
Benzyne Behaves as a Dienophile
Benzyne is a fairly reactive dienophile, and givesDiels-Alder adducts when generated in thepresence of conjugated dienes.